1. Field of the Invention
The present invention relates to methods and apparatuses for forming multi-layered circuit patterns widely used in electronic devices.
2. Description of the Related Art
A multi-layered circuit board having a multi-layered circuit on which semiconductor devices, including a large-scale integration (LSI) circuit, and various electronic parts are mounted is now widely used as the heart of electronic devices, communication devices, computers, and the like. A typical substrate of a multi-layered circuit board to be used for such purposes is made of a composite material containing a reinforcing material (e.g., ceramic or fiberglass) and plastic (e.g., epoxide resin). Some types of substrate for a circuit board to be mounted in compact equipment, such as mobile phones and cameras, are made of a flexible material (e.g., polyester resin or aramid resin) that can contribute to improved mountability of the circuit board. Recently, as the size of electronic devices decreases and their board density increases, multi-layered circuit boards with 8 or 16 layers have become mainstream while, previously, most circuit boards were single-side and double-side boards. At the same time, as the operating speed of electronic circuits increases, the fineness and density of circuit patterns are increasing rapidly.
There are various methods for forming a circuit pattern on a circuit board. For example, Japanese Patent Laid-Open No. 11-163499 discusses a method in which a conductive pattern forming solution (which exhibits conductivity) and an insulating pattern forming solution (which exhibits insulation properties) are simultaneously ejected from a liquid ejection head, such as an inkjet recording head, onto the surface of a substrate, on which a conductive pattern and an insulating pattern are drawn to create a complete circuit pattern layer, which is then stacked on top of one another to form a multi-layered circuit. However, since this method causes the mixing of the conductive pattern forming solution and the insulating pattern forming solution at the boundary between these solutions and causes smearing of the circuit pattern, it is difficult to achieve a fine and high-density circuit pattern.
However, to achieve a high-density circuit board, it is essential to stack multiple layers of circuit patterns. A liquid ejection method has an advantage over a known subtractive process in that the liquid ejection method can facilitate the stacking of multiple layers, as it can allow the formation of a circuit pattern with a thickness of only several to several tens of micrometers. However, the amount of droplets ejected from each of a plurality of nozzles in a liquid ejection head varies. For example, as shown in FIG. 8A to FIG. 8C, the nozzle configuration in a nozzle assembly 1003 of a liquid ejection head 1002 affects the distribution of droplets 1004 and causes variations in the thickness of a circuit pattern 1005 formed on a substrate 1001. In most cases, variations in the amount of the droplets 1004 occur in the process of manufacturing the liquid ejection head 1002 and are caused by multiple factors. Some factors may cause a random distribution of the droplets 1004 regardless of the nozzle configuration, whereas other factors may cause the droplets 1004 to be distributed as shown in FIG. 8A, 8B, or 8C. In FIG. 8A, the amount of the droplets 1004 at each end of the nozzle row is smaller than that in the middle thereof. In FIG. 8B, the amount of the droplets 1004 in one-half of the nozzle row is smaller than that in the other half thereof. In FIG. 8C, the amount of the droplets 1004 gradually increases from left to right along the nozzle row. In a liquid ejection head used in typical printers and apparatuses for creating circuit boards, variations in the amount of droplets ejected from different nozzles are normally 20% or less, which has not caused any problems to date in producing a multi-layered circuit board with four layers or less.
However, such variations in the amount of droplets cannot be accommodated in a multi-layered circuit with ten or more layers, which is becoming mainstream in recent years. FIG. 3A and FIG. 3B illustrate a multi-layered circuit board produced by stacking multiple circuit patterns formed with a liquid ejection head having the above-described configuration which causes variations shown in FIG. 8A. For easy understanding, FIG. 9A and FIG. 9B illustrate a four-layered circuit board produced by sequentially stacking the first through fourth layers with a liquid ejection head which can cause considerable variations, which are as high as 50%, in the amount of droplets ejected from different nozzles. This means that, in this circuit board, each layer has a level difference of about half the thickness thereof. As the stacking process proceeds, a level difference in each layer accumulates to a considerable level in the resulting four-layered circuit board. Generally, no significant problems arise if this resulting level difference is smaller than the thickness of a single circuit pattern layer. However, if a level difference which is larger than the thickness of a single circuit pattern layer is produced in a conductive pattern, the conductive pattern will be cut, and the circuit can be easily broken. If a level difference which is larger than the thickness of a single circuit pattern layer is produced in an insulating pattern, the resulting poor insulation or short circuit between patterns can lead to a critical failure in the circuit board. As described above, in a liquid ejection head generally used, variations in the amount of droplets ejected from different nozzles are normally 20% or less. This causes no problems in a four-layered circuit board, as a level difference in the four-layered circuit board is equal to or smaller than the thickness of a single layer. However, in a circuit board with five or more layers, it is highly likely that a failure in a circuit pattern occurs, as a level difference in such a circuit board is larger than the thickness of a single layer.
To produce a circuit board which extends over the length of the nozzle row of the liquid ejection head 1002 in FIG. 8A to FIG. 8C, first, as shown in FIG. 10A, the circuit pattern 1005 corresponding to the length of the nozzle row is formed in a drawing area. Next, as shown in FIG. 10B, the liquid ejection head 1002 is moved by the length of the nozzle row to form a circuit pattern 1006 in the next drawing area. The cross-sectional profile of each layer of the circuit pattern 1006 is the same as that of the circuit pattern 1005, in which a considerable level difference is produced.
Variations in the thickness of such a circuit pattern are caused not only by variations in the amount of the droplets 1004 ejected from different nozzles. As shown in FIG. 10C, unevenness on the surface of the circuit pattern 1005 is produced when the direction of the droplets 1004 ejected from nozzles vary among the nozzles.
Again, in most cases, variations in ejection direction are caused by problems in the process of manufacturing the liquid ejection head 1002. As in the case of variations in the amount of the droplets 1004, variations in ejection direction can occur randomly regardless of the nozzle configuration. There may be other cases where variations in ejection direction occur due to the configuration of specific nozzles in the nozzle row.